No_5_February_1962 — Page 71

Dry density

(lbift 3)

* }

Table 1

AUTOCLAVED AERATED CONCRETE STRENGTH AND ELASTICITY

Approximate

compressive strength

(tested west)

(lb/ft2)

Modulus of elasticity

(Iblin 2 x 10o.}

25

200

30

300

0.21

35

400

0.27

40

500

0.33

600

0.39

45

50

solubility of the lime, and saponin or other such substance to stabilize the gas cells. When cement is used, soluble alkali, such as caustic soda, may be added to initiate the reaction with the metal powder.

The sand, burnt shale or slag is ground to a fine powder (normally of the fineness of Portland cement). The mixing is batch-wise, one batch to each mould. The same mould can be used whether blocks or reinforced units are to be made, since the size and shape of the final product is determined by the subsequent wire-cutting of the large block obtained from a single casting. Where reinforced units are to be form- ed, the prefabricated reinforcement mats are accurately located in the mould, so that, when the casting is subsequently cut into slabs, the reinforcement will be in its proper position in each separate slab.

up of

The batch of slurry made cement (or lime), ground sand and the other ingredients, is run into the mould; this is only partly filled, but after about 20 minutes, as a result of gas formation, the mixture is sufficiently aerated to fill it to overflowing. After about 6 hours the mass is igid enough to withstand cutting. At this stage the sides of the mould are removed and the casting is cut by parallel wires and others at right-angles to them, so as to produce blocks or slabs of the required dimen- sions, after which the whole is transfer- red to the autoclave for high-pressure steam-curing. The autoclaves are often about 10 feet in diameter and more than 80 feet long, and the contents are main- tained at a pressure of about 150 lb/in* pressure for 14-18 hours. After removal from the autoclave the product requires no further curing

PROPERTIES

Density and strength

In situ aerated concrete is often made in very low densities (10-12 lb/ft3), for use where good thermal insulation is required but strength is unimportant. Autoclaved aerated concrete, however, is usually made in the dry density range

700

0.45

25-50 lb/ft3. The lightest units in this range are normally used for slab-form insulation; their strength is low, but sufficient in some cases for non-load- bearing partitions. The intermediate and higher density products have suf- ficient strength for loadbearing purposes: structural reinforced aerated concrete is used at densities between 30 and 40 ib/ft3, that is, one-fifth to one-quarter of that of conventional concrete.

Table I gives a general indication of the compressive strengths to be expected from autoclaved aerated concrete of different densities within the density range 25-50 lb/ft3. At present there is no British Standard covering the strength of aerated concrete units (BS.1364 relates to dimensions only) but the minimum strength specified in BS.2028 for lightweight-aggregate build- ing blocks is 400 lb/in2, and Table 1 shows that this would be met by autoclaved acrated concrete units hav- ing a density of 35 lb/ft and over.

Modulus of elasticity

The modulus of elasticity of aerated concrete is much lower than that of conventional concrete. The mean figures from a number of laboratories are given in Table 1. For ordinary concrete, the modulus of elasticity is normally be tween 3×10 and 5×10 lb/in2, i.e. about 10 times greater

Creep

Aerated concrete, like cther cement products, undergoes plastic deformation, or creep, under sustained loads. Very

Material

little is yet known about this property of the material, but what work has been done indicates that the effect of creep at working loads is not greater than with ordinary dense concrete.

Drying shrinkage

All cement products undergo changes in volume when subjected to changes in moisture conditions, and although the actual volume changes are not large they are of great importance in terms of the functional behaviour of the concrete or mortar. Neat cement paste shows very high drying shrinkage, but in general a hard, rigid aggregate, such as is used in dense concrete, restrains the shrinkage of the cement and so of the concrete made from it. Aerated concrete usually has no aggregate other than a fine filler, which is generally present in much smaller proportion than the aggregate in concrete, so that aerated concrete, if given only ordinary air-curing, has a high drying shrinkage. Autoclaving, however, fundamentally alters the con- stitution of the binder and 80 may reduce the shrinkage to about one-third of what it would have been with air- curing. This is shown in Table 2.

Thermal insulation

ot

One of the principal features aerated concrete is its low thermal conductivity, this being roughly propor- tional to the density of the material. Table 3 shows typical k-values of different grades of aerated concrete in comparison with the ordinary dense

concrete.

The thermal conductivity of materials increases with their moisture content. The k-values quoted relate to specimens in equilibrium with the humidity of the ambient air and thus having a moisture content, presumably, of about 2% by volume. Aerated concrete in use as the inner leaf in cavity walls facing a centrally heated room would have a moisture content of this order, but in the outer leaf the moisture content

might rise to ΙΟ per cent ог more, according to the conditions of exposure, thus reducing the insulating value of the leaf. Available data indicate that the k-value of bone-dry material is increased by a factor of 1.30 for 1% moisture by volume, 1.75 for 5% and 2.10 for 10%.

Table 2

DRYING SHRINKAGE

Drying shrinkage (%)

Air-cured

Autoclave-cured

Aerated concrete A

0.17

B

0.22

0.06

0.07

THE HONG KONG & FAR EAST BUILDER-VOLUME 16, NUMBER 5

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